787 electrical system - variable frequency generators?
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I am a former ramp and tech support engineer just looking in, I like to keep an eye on whats going on and have been looking for info on the strange hi volts DC I keep hearing about.
It seems that the +/- 270v 'DC' is of course actually AC, the square wave being very much easier, and cheaper, to generate electronically from rectified frequency wild AC.
They seem to be using this level of control to give high power motors, I assume they are induction, higher starting torque with the initial lower frequency.
It may be this basic or it could be a whole lot more complicated!
Square wave AC is also very messy, being every harmonic possible if I remember correctly, and that can interfere with delicate digital devices, like the rest of the plane.
Frequency wild is alright for galleys that use a lot of power in quiet cruise when other demands may be lower, they are resistive loads that are not greatly frequency sensitive and are designed for normal cruise power settings.
Having been on a couple of Boeing courses, 747-400 and 777, we found the instructor could be very insistent on some matters. We resolved it to 2 reasons.
First.B. S. E. Boeing Standard English was sometimes the cause, they are often teaching engineers for whom English (full of ambiguities) is a second, or even third, language. BSE is very straight forward, but sounds rather awkward to us
The other reason was Boeing proprietary, meaning they had some patent involved, or hoped to. Then the instructors would say quietly that there were 'lawyers involved' and we shut up, knowing that discussion was useless.
It seems that the +/- 270v 'DC' is of course actually AC, the square wave being very much easier, and cheaper, to generate electronically from rectified frequency wild AC.
They seem to be using this level of control to give high power motors, I assume they are induction, higher starting torque with the initial lower frequency.
It may be this basic or it could be a whole lot more complicated!
Square wave AC is also very messy, being every harmonic possible if I remember correctly, and that can interfere with delicate digital devices, like the rest of the plane.
Frequency wild is alright for galleys that use a lot of power in quiet cruise when other demands may be lower, they are resistive loads that are not greatly frequency sensitive and are designed for normal cruise power settings.
Having been on a couple of Boeing courses, 747-400 and 777, we found the instructor could be very insistent on some matters. We resolved it to 2 reasons.
First.B. S. E. Boeing Standard English was sometimes the cause, they are often teaching engineers for whom English (full of ambiguities) is a second, or even third, language. BSE is very straight forward, but sounds rather awkward to us
The other reason was Boeing proprietary, meaning they had some patent involved, or hoped to. Then the instructors would say quietly that there were 'lawyers involved' and we shut up, knowing that discussion was useless.
It seems that the +/- 270v 'DC' is of course actually AC, the square wave being very much easier, and cheaper, to generate electronically from rectified frequency wild AC.
They seem to be using this level of control to give high power motors, I assume they are induction, higher starting torque with the initial lower frequency.
.
They seem to be using this level of control to give high power motors, I assume they are induction, higher starting torque with the initial lower frequency.
.
Not so much on aircraft. This is true for automotive alternators. But the inability to disconnect the excitation (field) would leave a generator continually feeding into a short circuit. And for a generator capable of producing hundreds of kilowatts, that's a major fire hazard.
The overall system might be considered to be self excited due to the PMG (permanent magnet generator) source. But the output of that is routed through the GCU (generator control unit) which provides field control for voltage regulation of the main generator and a relay to disconnect it for fault clearing. The PMG itself is a low energy generator and designed so that if it shorts internally, the energy dissipated is tolerable.
The overall system might be considered to be self excited due to the PMG (permanent magnet generator) source. But the output of that is routed through the GCU (generator control unit) which provides field control for voltage regulation of the main generator and a relay to disconnect it for fault clearing. The PMG itself is a low energy generator and designed so that if it shorts internally, the energy dissipated is tolerable.
Not so much on aircraft. This is true for automotive alternators. But the inability to disconnect the excitation (field) would leave a generator continually feeding into a short circuit. And for a generator capable of producing hundreds of kilowatts, that's a major fire hazard.
The overall system might be considered to be self excited due to the PMG (permanent magnet generator) source. But the output of that is routed through the GCU (generator control unit) which provides field control for voltage regulation of the main generator and a relay to disconnect it for fault clearing. The PMG itself is a low energy generator and designed so that if it shorts internally, the energy dissipated is tolerable.
The overall system might be considered to be self excited due to the PMG (permanent magnet generator) source. But the output of that is routed through the GCU (generator control unit) which provides field control for voltage regulation of the main generator and a relay to disconnect it for fault clearing. The PMG itself is a low energy generator and designed so that if it shorts internally, the energy dissipated is tolerable.
I should have written
"Modern AC generators with field coils are usually self-excited via PMG"
Sorry
Just a general point - and I don't know if it applies here, but "square wave DC" could describe a series of +270 Volts "Direct Current" pulses, with an equal mark space ratio. In other words, the voltage is switched on/off/on/off etc. between +270VDC and zero volts.
True "alternating current", AC, alternates the polarity every half cycle; so on the first half cycle the voltage rises from zero volts to maximum positive voltage and back to zero. In the following half cycle the voltage changes polarity and goes to maximum negative voltage and back to zero again. The AC wave form is symmetrical around zero volts, i.e. it goes from zero - max positive volts - zero - max negative volts, and repeats. What 'Square wave DC' means to me that it is only ever between zero and positive voltage.
Normal AC voltage is a sine wave, owing to being generated by a physically rotating generator. Square wave is produced by electronic switches, and goes from zero to maximum (near) instantaneously, giving the classic square shaped waveform.
Many DC motors now work from square wave DC. This gives higher starting torque, good low speed response and lower heat in the windings. The mark-space ratio can be modulated for varying loads on the motor; the 'on' time of the square wave being very brief for light loads, much longer for heavy loads.
Your battery drill probably uses variable mark-space ratio square wave DC drive, and on some you can hear the high frequency of the square wave generator as you slowly pull the trigger.
True "alternating current", AC, alternates the polarity every half cycle; so on the first half cycle the voltage rises from zero volts to maximum positive voltage and back to zero. In the following half cycle the voltage changes polarity and goes to maximum negative voltage and back to zero again. The AC wave form is symmetrical around zero volts, i.e. it goes from zero - max positive volts - zero - max negative volts, and repeats. What 'Square wave DC' means to me that it is only ever between zero and positive voltage.
Normal AC voltage is a sine wave, owing to being generated by a physically rotating generator. Square wave is produced by electronic switches, and goes from zero to maximum (near) instantaneously, giving the classic square shaped waveform.
Many DC motors now work from square wave DC. This gives higher starting torque, good low speed response and lower heat in the windings. The mark-space ratio can be modulated for varying loads on the motor; the 'on' time of the square wave being very brief for light loads, much longer for heavy loads.
Your battery drill probably uses variable mark-space ratio square wave DC drive, and on some you can hear the high frequency of the square wave generator as you slowly pull the trigger.
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According to the diagram TURIN posted, the waveform is +/-270V symmetrical around zero, (rather than 0 to +270) so wouldn't that qualify is as AC under your distinction?
Probably, if that is the case, but I am not an authority on definitions.
Hhm I may be wrong but I think +/- 270 V DC is pure DC. No square wave, no alternating polarity.
Look at the colours of the schematic by Turin.
But I don't get the +/-
Look at the colours of the schematic by Turin.
But I don't get the +/-
